1. Field of the Invention
This invention relates to a signal recording and reproducing apparatus adapted to record and reproduce data on and from an opto-magnetic disk. More particularly, it relates to such a device by means of which new signals can be recorded within a time interval apparently equal to a time interval required for signal reproduction.
2. Description of the Prior Art
In general, an opto-magnetic recording system is attracting attention in recent years in that data rewriting is feasible with this system in distinction from other optical recording systems such as what is called a direct read after write system (DRAW system). Above all, research is being conducted in the field of practical application of such system along with the opto-magnetic disk as an external computer memory, with a view towards future commercialization.
The operating principle of the conventional opto-magnetic recording system shown in FIG. 1 is basically as follows. First of all, a downwardly directed uniform magnetization M is established in a magnetic recording medium 101, such as magnetic disk, as shown in FIG. 1 (a). Then, as a weak bias magnetic field H opposite in direction to the magnetization M is applied from the outside, as shown at (b) in FIG. 1, by a magnetic coil 103 or a permanent magnet, not shown. The disk is irradiated by a light beam, usually a laser beam LB, modulated in accordance with the data signals to be recorded on the disk, for locally elevating the temperature of the irradiated disk portion. This causes the direction of magnetization of the irradiated disk portion to be reversed into coincidence with the direction of the bias field H so that there remains a recorded magnetization M' as shown in FIG. 1 (c). During signal reproduction, the well-known effect of magnetic rotation or the Faraday effect concerning the interaction between the light and the magnetism is utilized so that, when the laser beam sweeps the magnetic recording medium 101, the different angle of rotation of the polarization plane of the laser beam with the different directions of magnetization M or M' is sensed for optically reading the recorded data signals.
When recording the data signals on the previously recorded magnetic medium 101, the laser beam is applied generally on the overall surface of the magnetic medium or to the portion of the medium where the re-recording is desired, for aligning the direction of magnetization of the irradiated portion with the preset constant direction, at this time, the direction of the externally applied bias magnetic field coincides with the direction of the magnetization M. In this manner, the reverse magnetization M' is rearranged to be in alignment with the original direction of magnetization M so that the previously recorded data are erased. Next, the direction of the external magnetic field or bias magnetic field is reversed from that of magnetization M, as in the case of the above described recording operation, and the head unit is caused to sweep the magnetic medium as the laser beam modulated in accordance with the data signals to be re-recorded is irradiated on the magnetic medium.
In the opto-magnetic recording system, as described hereinabove, it is necessary to first erase the old data on the disk with a magnetic field applied thereacross and then to write the new data with the direction of the magnetic field reversed. This causes an inconvenience in that, in contrast to the case of the magnetic tape or the magnetic disk, the magnetic erasure and recording operations cannot be performed simultaneously. Thus, when writing the new data, the time equal to that used for recording will be consumed for erasure, so that the recording time is equal to twice the reproducing time.
Therefore, should the data to be recorded be continuous signals such as music source, real time recording of these signals is not feasible so that the change of recording is lost once and for all, which is a disadvantage as compared to the conventional recording and reproducing system.